Lab-scale experiments have been carried out to gain more insight in the mechanisms responsible for the volatilisation of heavy metals during CCA wood waste pyrolysis. A step-wise approach has been followed, starting with mixtures of wood model compounds-lignin and glucose- and arsenic model compounds -arsenic pentoxide (As2O5) and chromium arsenate (CrAsO4)- and applying thereafter these insights to CCA wood waste pyrolysis. Samples were pyrolysed in an updraft fixed bed reactor at various gage pressures (0-5 bar) and till various temperatures (310°C-390°C). After pyrolysis the samples were analysed by inductively coupled plasma-mass spectrometry (ICP-MS), to determine the amount of heavy metals still present in the solid pyrolysis residue. Moreover a ratio method was used to calculate arsenic and chromium retentions in pyrolysed CCA wood samples in order to reduce the experimental uncertainty.
Elevated pressure resulted in higher metal retentions in all model compound mixtures, except for lignin/As2O5 mixtures, while metal retentions are on average little affected by temperature in the range of 310°C to 370°C. Arsenic retention in glucose/As2O5 mixtures is substantially higher compared to lignin/As2O5 mixtures. Probably arsenic volatilisation in As2O5 samples is mainly controlled by desorption of arsenic trioxide (As2O3) from the solid residue and subsequent entrainment by pyrolysis gasses.
Average arsenic retention in lignin/CrAsO4 samples is higher compared to lignin/As2O5 mixtures but lower compared to glucose/As2O5 mixtures while CrAsO4 decomposition already starts at 310°C. Average metal retention in glucose/CrAsO4 mixtures is lower compared to glucose/As2O5 mixtures and lignin/CrAsO4 mixtures. Apparently, due to a better binding to lignin, CrAsO4 is better stabilised on the lignin pyrolysis residue. In the case of glucose, arsenic and chromium species are poorly retained in the pyrolysis residue, leading to higher metal volatilisation in glucose samples.
Metal retentions during pyrolysis of real CCA wood samples are higher at higher pressure while, between 330°C and 370°C, retention decreases with increasing temperature. In this temperature range average arsenic retention attains 94% at 0 bar and 99% at 5 bar and average chromium retention is 97% at 0 bar and 96% at 5 bar. Metal retention in CCA wood samples is higher compared to CrAsO4 experiments. This is believed to be due to better fixation of CrAsO4 to the wood substrate and due to formation of agglomerates, consisting of thermally stable compounds like calcium arsenate (Ca5(AsO4)3(OH)).